Exhaust devices, such as exhaust hoods and ventilated ceilings, are used to remove pollutants from occupied spaces with sources of pollutants. Examples include factories, kitchens, workshops, and food courts which contain industrial processes, kitchens appliances, tools, and portable cooking appliances, respectively. Preferably, exhaust hoods remove pollutants by drawing them from a collection area near the source and may also provide a containment function, usually by ensuring that the velocity of exhaust is sufficient near the source to overcome any local buoyancy or draft effects to ensure that all pollutants are prevented from escaping to the general occupied space. By managing transients in this way, an effective capture zone is provided.
In exhaust systems, an exhaust blower creates a negative pressure zone to draw pollutants and air directly away from the pollutant source. In kitchen applications, the exhaust generally draws pollutants, including room-air, through a filter and out of the kitchen through a duct system. A variable speed fan may be used to adjust the exhaust flow rate to match the extant requirements for capture and containment. That is, depending on the rate by which the effluent is created and the buildup of effluent near the pollutant source, the speed of exhaust blower may be manually set to minimize the flow rate at the lowest point which achieves capture and containment.
The exhaust rate required to achieve full capture and containment is governed by the highest transient load pulses that occur. This requires the exhaust rate to be higher than the average volume of effluent (which is inevitably mixed with entrained air). Such transients can be caused by gusts in the surrounding space and/or turbulence caused by plug flow (the warm plume of effluent rising due to buoyancy). Thus, for full capture and containment, the effluent must be removed through the exhaust blower operating at a high enough speed to capture all transients, including the rare pulses in exhaust load. Providing a high exhaust rate—a brute force approach—is associated with energy loss since conditioned air must be drawn out of the space in which the exhaust hood is located. Further, high volume operation increases the cost of operating the exhaust blower and raises the noise level of the ventilation system.
Also known are “make up” air systems, some of which have been proposed to be combined with exhaust hoods in a manner in which make-up air is propelled toward the exhaust intake of a hood. This “short circuit” system involves an output blower that supplies and directs one, or a combination of, conditioned and unconditioned air toward the exhaust hood and blower assembly. Such “short circuit” systems have not proven to reduce the volume of conditioned air needed to achieve full capture and containment under a given load condition.
Another solution in the prior art is described in U.S. Pat. No. 4,475,534 titled “Ventilating System for Kitchen.” In this patent, the inventor describes an air outlet in the front end of the hood that discharges a relatively low velocity stream of air downwardly. According to the description, the relatively low velocity air stream forms a curtain of air to prevent conditioned air from being drawn into the hood. In the invention, the air outlet in the front end of the hood assists with separating a portion of the conditioned air away from the hood. Other sources of air directed towards the hood create a venturi effect, as described in the short circuit systems above. As diagramed in the figures of the patent, the exhaust blower must “suck up” air from numerous air sources, as well as the effluent-laden air. Also the use of a relatively low velocity air stream necessitates a larger volume of air flow from the air outlet to overcome the viscous effects that the surrounding air will have on the flow.
In U.S. Pat. No. 4,346,692 titled “Make-Up Air Device for Range Hood,” the inventor describes a typical short circuit system that relies on a venturi effect to remove a substantial portion of the effluent. The patent also illustrates the use of diverter vanes or louvers to direct the air source in a downwardly direction. Besides the problems associated with such short circuit systems described above, the invention also utilizes vanes to direct the air flow of the output blower. The use of vanes with relatively large openings, through which the air is propelled, requires a relatively large air volume flow to create a substantial air velocity output. This large, air volume flow must be sucked up by the exhaust blower, which increases the rate by which conditioned air leaves the room. The large, air volume flow also creates large scale turbulence, which can increase the rate by which the effluent disperses to other parts of the room.
Currently, in workplaces where fumes, dust, or chemical vapors present a hazard, local exhaust ventilation devices are used to prevent workers from inhaling contaminated air. Generally, an exterior exhaust hood, for example, a receiving hood, is disposed above the emission source to remove airborne contaminants. However, theoretical capture efficiency of such a receiving hood holds only in still air, the capture efficiency decreases due to crosswind in the surrounding environment, no matter how weak the crosswind is. To control the adverse effect of crosswind, a fume hood having a back panel, two side panels, and a hood sash in the front has been designed to replace a receiving hood. However, the side panels and hood sash of a fume hood limit the size of operation space for operators' upper limbs. Therefore, how to eliminate the adverse effect of crosswind, and meanwhile retain the freedom of operators' upper limbs, becomes a key topic to a receiving hood.
In order to accomplish the key topic, U.S. Pat. No. 4,788,905, published on Dec. 6, 1988, disclosed a combination cooking, heating and ventilating system. The system contains an open fire grill surrounded by an unperforated griddle, both of which are surrounded by an eating counter. A fan is positioned below the cooking grill and griddle which forces the air upward between the eating counter and the griddle in the shape of an air curtain for removing hot smoking air from the cooking area. However, due to the limited size, the fan is not applicable in a large-scale worktable. Further, generally speaking, there is not necessarily enough space to accommodate the fan device below the worktable.
U.S. Pat. No. 5,042,456, published on Aug. 27, 1991, disclosed an air canopy ventilation system. The system comprises a surface having two substantially parallel spaced apart side panels surmounted at their respective upper edges by a canopy. A vent means having a plurality of outlets extends between the side panels and substantially the whole length of the front edge of the surface. A fan means connected to the vent means is adapted to drive a flow of air through the vent means upwardly to form a curtain of air over the front of the system, thereby entraining within the area fumes and odors. The upwardly flowing air, fumes and odors are removed by an exhaust means. Though the system can solve the problem of the lateral diffusion of the smoke and the influence of the crosswind, the air flow perpendicular to the side panel affects the efficiencies of the upward air curtain and canopy. Meanwhile, the structure of the system having the side panel and back panel limits the size of the operation space in which the operator can operate.
Further, U.S. Pat. No. 6,450,879, published on Sep. 17, 2002, disclosed an air curtain generator includes a casing with a fan received therein so as to blow an air curtain from opening of the casing, and the air curtain separates the workers and the source where generates contaminated air. However, the air curtain only isolates the smoke from laterally diffusing towards the operator, but does not isolate the smoke from diffusing towards the side without the air curtain generator. Additionally, the inventor of the present invention disclosed an air curtain generator in U.S. Pat. No. 6,752,144 published on Jun. 22, 2004, and the present invention is a continued invention along the lines of this patent.
In U.S. Pat. No. 685,121, an exhaust hood has a vertical curtain jet which helps to prevent the escape of pollutants in the vicinity of the source. U.S. Pat. Nos. 4,811,724 and 5,220,910 describe a canopy type exhaust hood with a horizontal jet to enhance capture. In one the latter, general ventilation air is provided on a side face of the canopy hood. U.S. Pat. No. 5,063,834 describes a system in which a ceiling-level ventilation zone is created to remove unducted fumes from exhaust hoods. U.S. Pat. No. 4,903,894 describes displacement ventilation techniques in which ventilation air is brought into a conditioned space at low velocity and without mixing to capture impurities and convey them toward a removal zone near the ceiling. U.S. Pat. No. 5,312,296 describes an exhaust hood that is located near the ceiling with an exhaust intake jutting from the ceiling level. Ventilation air enters the occupied space via a horizontal jet that runs along the ceiling level and a displacement ventilation registers that distributes air at low (non-mixing) velocities.